Airborne platforms such as observation satellites having an on-board camera are used to map geographic terrain. Proper registration of the camera image relative to the terrain is subject to various inaccuracies arising, for example, from instabilities in the camera characteristics, atmospheric variations, and changes in satellite orbit. In order to ensure accurate correlation between the mapped area imaged by the camera and the actual terrain, it is known to employ location markers whose coordinates relative to the earth are precisely known. When an aerial image containing such location markers is subsequently processed, the location markers enable the image to be accurately calibrated so that the location in space of all other features in the image may be accurately determined.
Clearly, for such an approach to be operable, it is essential that the location markers be visible to the airborne camera. In order for this requirement to be achieved when observation satellites are used, it is known to employ large static resolution targets, such as circular dishes shown in FIG. 1 having a diameter of between 2-10 meter whose center coordinate is accurately known. It is fairly simple to process the resulting image to determine the center of the circular image corresponding to the circular dish and thus to obtain the satellite camera calibration characteristics. However, owing to the large dimensions of the resolution targets, such location markers occupy a commensurately large area of land, which is thus unavailable for other uses. An additional drawback of such markers results from their being their static relative to the earth thus requiring that the satellite undertake special maneuvers to calibrate the camera and ensure that it properly tracks the satellite.
US2005/271301 to Rafael Armament Development Authority Ltd. discloses a method and system for providing a platform with pseudo-autonomous correlation between a perspective view generated by a platform-based imaging sensor and an orthogonal photographic representation. This requires registration of a perspective image with an orthogonal photographic representation which allows features in the perspective image to be associated with their geographic locations. Conventional techniques for performing the registration are used.
US2003/044085 discloses a method for photogrammetric block adjustment of satellite imagery using a simplified adjustment model in-lieu of a physical camera model. A known relationship between image space line and sample coordinate and object space X, Y, Z coordinates is provided by a mathematical model. Observations comprise ground control points, tie points, or other observations for which approximate knowledge of object and image space coordinates is available. No detailed description of the tie points is provided.
US2004/234123 discloses a surveying system that comprises a position relation calculating processor, a correspondence establishing processor, and an image processor is provided. The position relation calculating processor calculates a positional relation between a coordinate system to which measurement information of a measurement point refers and an image of a surveying field, where a staking point is included. The correspondence establishing processor establishes correspondence between three-dimensional position information of the staking point and two-dimensional position information of a point corresponding to the staking point on the schematic image. The image processor superimposes a symbol that indicates the position of the staking point on the schematic image in accordance with the above correspondence.
EP1662228 relates to a method and system for scanning a three-dimensional object comprising scanning a surface of the object from a first scanning position, generating a first scanning sample comprising a point cloud having data points corresponding to positions on the surface of the object determining the coordinates of the first scanning position by a satellite based positioning system, correcting the inaccuracy of the positioning system by using an additional error signal scanning the surface of a three-dimensional object from at least one further scanning position, generating at least one further scanning sample comprising such a point cloud determining the coordinates of the at least one further scanning position by the positioning system, again correcting the inaccuracy of the positioning system registering the first and the at least one further scanning samples based on the first and the at least one further scanning positions, such that the samples are oriented relatively to each according to the orientation of the respective surfaces of the three-dimensional object.
U.S. Pat. No. 6,680,693 (Urban et al.) discloses a method and apparatus to track the position of the sun and direct a static ground-mounted object to the direction of the sun. The object may include a solar collector, solar cell, or test panel, among other possible devices or applications requiring near continuous exposure to rays of the sun. The apparatus uses a GPS device to determine the position of the object on the earth. The apparatus includes a controller operatively coupling to the GPS device for calculating the relative position of the sun with respect to the object. The controller operatively couples to a positioning system that includes a first drive and a second drive coupled to the object. Commands from the controller operate the positioning system to articulate the object and automatically direct it towards the relative position of the sun.
Some of these references teach techniques relating to navigation and tracking that are generally applicable to the present invention and to this extent there complete disclosures are incorporated herein by reference. None of them addresses the need to provide compact location markers, which are clearly visible to a satellite imaging system so as to allow precise calibration of their spatial coordinates. Nor do they appear to relate to mobile location markers that avoid the need for satellite adjustment.
It would be preferable if the location markers were more compact so as to occupy less land, while nevertheless being clearly visible to the satellite imaging system and yet allowing precise calibration of their spatial coordinates. It would likewise be preferable that they be mobile to avoid the need for satellite maneuvers to calibrate the camera.